A vehicle collision avoidance radar is a radar mounted on a vehicle and is used to detect a distance to a target, which includes a vehicle traveling ahead, as well as a direction and a relative speed of the target. A signal transmitted from the radar mainly uses a millimeter wave frequency band.
A transmitting antenna and a receiving antenna are provided separately in the vehicle collision avoidance radar, where two or more channels are typically provided for the receiving antenna. The vehicle collision avoidance radar emits radio waves from the transmitting antenna, receives a reflected wave from a target by the receiving antenna, and performs signal processing. At this time, the distance can be obtained from the time required for the reflected wave to return, the direction can be obtained from a phase difference between the receiving channels, and the relative speed can be obtained from the frequency of the reflected wave.
A desirable characteristic of the vehicle collision avoidance radar is to be able to detect a target as far ahead as possible and to perform detection as wide as possible to the sides in order to deal with an object popping out from the side, for example. That is, a desirable characteristic of the antenna includes a high front gain and no presence of a null out to a wide angle. Note that the null refers to a point with the minimum gain in an antenna radiation pattern such as a boundary between a main lobe and a side lobe, and increasing the gain of the null is called null filling.
An antenna device (hereinafter referred to as a “conventional antenna device” as appropriate) described in Patent Literature 1 is known as an antenna device performing null filling. The conventional antenna device includes four radiating elements arrayed in a direction perpendicular to the ground and a feeder circuit that feeds high frequency signals to the four radiating elements where, when the radiating elements are numbered in order from the top as element 1, element 2, element 3, and element 4, power is fed to an upper radiating element group (element 1 and element and a lower radiating element group (element 3 and element 4) with an unequal division ratio.
As illustrated in FIG. 2 and FIG. 3 of Patent Literature 1, the conventional antenna device has the radiation pattern in which radiated fields from an even number of radiating elements (four in the literature) are added in phase to obtain the maximum gain (0 dB) at 0 degree, or in the front direction of an antenna plane. On the other hand, in the vicinity of 17 degrees within a vertical plane, element 1 is opposite in phase to element 3 while element 2 is opposite in phase to element 4, so that a vector sum of the four radiated fields is the minimum, or becomes a first null.
The literature describes that, in the conventional antenna device, the gain of the first null is substantially equal to 0 (=30 dB or less) when power is fed to the upper radiating element group and the lower radiating element group with an equal division ratio (1:1), but when power is fed with an unequal division ratio (1:2 to 1:4), the vector sum of the four radiated fields does not equal 0 so that the gain of the first null increases to about −18 dB to −12 dB.